1. Field of the Invention
The present invention relates to a silver halide photographic emulsion sensitized with dyes and, more particularly, it relates to a silver halide photographic emulsion having high sensitivity to "flash exposure" by a blue-green light and having high storage stability under high temperature and high humidity. The term "flash exposure" will be defined hereinafter in this specification.
2. Description of the Prior Art
Recently, new techniques have been developed for rapid communication of information and recording. For example, the press facsimile system for sending newspaper copies to a distant place, high speed photographic typesetting systems for composing type extremely rapidly, cathode ray tube display systems for indicating and recording the output information from a computer as figures or letters using a cathode ray tube, and like systems. The light-sensitive material used in these systems is exposed for a period of less than 1/10,000 second, often as short as about 1/1,000,000 second, by means of an exposing apparatus provided in the above-described systems.
As the light source or exposing apparatus, there are combinations of high illuminance light sources such as a xenon flash, a flying spot of cathode rays traced on a fluorescent tube, (as used in television, facsimile and the like), a laser, a xenon arc light or a high pressure mercury lamp with a high speed shutter, and the like.
The xenon flash light and the arc light have strong energy over a comparatively wide range of wavelengths. In these cases the light of short wave-length is largely absorbed in the optical apparatus in the system used, and hence the light of longer wave length relatively increases in strength.
In the cathode ray tube display, the luminescence from many kinds of fluorescent materials, e.g., P-1, P-11, P-15, P-16, P-22D, P-24, P-31 and P-22 has been utilized. Of these, it is known that the peaks of the spectral energy distribution of the luminescence of P-15, P-24 and P-31 are at about 505 nm, 520 nm and about 520 nm respectively. A silver halide photographic emulsion to be exposed to such blue-green light must be particularly sensitized with dyes.
The term "flash exposure" as is used herein means high-intensity short-time exposure shorter than about 1/1,000 second, preferably shorter than 1/10,000 second, which is appropriate for the above-described purposes.
The sensitization of silver halide photographic emulsions used in light-sensitive materials particularly adapted to flash exposure is influenced not only by the pAg of the emulsion, the halogen composition, grain size and the crystal habit, but also by the conditions of the chemical sensitization or of chemical ripening.
The dye sensitization of the emulsion to flash exposure has so far been said to be dominantly dependent on the light-sensitive characteristics of the silver halide grain itself contained in the emulsion. However, it has also been found that dye sensitization is largely dependent on the chemical structure and the properties of the dye used (see, for example, Japanese patent application Nos. 104009/69; 2819/70; and 12306/70). In addition, the supersensitization thereof to flash exposure is also dependent on the combination of the dye groups used (see, Japanese patent application Nos. 48546/70 and 90950/70). The functional mechanism thereof has not been clarified (see, for example, Japanese patent application No. 1320/71). Therefore, it cannot be concluded that conventional sensitizing dyes can be directly applied to sensitization to flash exposure.
Recently, two or three dyes in combination providing high sensitivity to flash exposure by blue-green light were found. However, most of these dye groups are so unstable under high humidity that when left for about 2 days under a relative humidity of 80% the sensitizing property thereof was reduced to less than 1/2.
It is convenient for silver halide light-sensitive materials to be capable of being treated under a safe light which is as bright as possible. Light-sensitive materials for flash exposure are desirably treated under an orange-yellow safe light such as No. 2A or No. 2B safe light manufactured by the Fuji Photo Film Co., Ltd. These safe lights absorb light of a wave-length shorter than about 540 nm. Accordingly, sensitizing dyes capable of providing spectral sensitivity distribution where maximum sensitivity lies in the wave-length region shorter than 520 nm and wherein sensitivity in the longer wave-length region is sharply reduced are very useful. However, dyes satisfying such conditions are very few.